WSFS doesn't have a permanent physical office. I suppose the closest thing to it might be the home of the current Chair or Treasurer, that being where the records are kept, but that's unsatisfying to people trying to squeeze WSFS into a form they can understand. WSFS doesn't run Worldcons. Worldcon committees do that. Those committees generally are legal, incorporated entities, in a way that WSFS is not.
Even the question of "When was WSFS formed?" isn't easily answered. I'm currently answering that question "1963," based on an article by George Scithers explaining how the document identifiable as the ancestor of today's WSFS Constitution was adopted. Prior to 1963, there were various references to WSFS, and an incorporated WSFS Inc that Plunged All Fandom Into War. Worldcons sort of muddled along, Eastercon-style, before that, and to some extent they still do today, but the structure adopted in 1963 is more or less the same thing we use today, modified by the creation of what is currently called the Mark Protection Committee in 1983.
My theory is that, for purposes of interacting with the rest of the world, individual Worldcon committees are licensed the right to use the WSFS service marks (Worldcon, Hugo Award, etc.) in conjunction with operating a World Science Fiction Convention in a specific year. The license comes from WSFS (by way of the site selection mechanism) and the marks are managed by the WSFS MPC. There is no required license fee payment, although most Worldcon Operating Committees make voluntary contributions to the MPC that could be considered an equivalent to a license fee. The whole structure is maddeningly vague to people (especially governmental bodies like those that issue trade/service mark registrations).
For most fans, this is all way too structured and formal. For most governmental agencies, it's much too unstructured and informal. And therefore, there is no single right answer to any questions about WSFS structure!